In standard polishing processes, the wafer surface is pressed against a polishing pad which is coated with a slurry containing a fine abrasive, such as silica. The polishing pad is usually flat, and the abrasive is held by the pores of the pad and mechanically scrapes away the high points of the wafer. In chemical-mechanical polishing (CMP), an acid or base is added to the slurry, which preferentially etches one or more of the materials to be removed. In some cases, the acid or base converts the material to be removed to a different form which is more readily removed by the abrasive.
In recent years, chemical-mechanical polishing has received more attention and played a more important role in the fabrication of microelectronic devices, mainly because of its capability of providing good global planarity, as well as good local planarity. If planarization processes are not applied in the device fabrication, the interconnect layers are typically finished with varying degrees of non-planarity. Such topography makes it difficult for the fabrication of the next layer. For example, a photoresist layer formed over a highly non-planar surface will not have a uniform thickness, which will lead to non-uniformity in the patterning of the resist. In the case of stacked patterns, this obstacle becomes even more prominent when such non-planarity is imprinted or amplified into the next layer, and the cumulative non-planarity may become so severe after just a few layers that the photolithography process becomes a limiting factor in the device fabrication. CMP processes, such as oxide and metal polishing, have been employed in the manufacturing of integrated circuit (IC) chips, but not so much in chip packaging.
While standard polishing and chemical-mechanical polishing processes can greatly improve planarity of a wafer, the processes do have particular drawbacks. For example, large areas of the wafer having relatively soft, easily abraded material can be over-etched (so-called "dishing"). Also, it is often difficult to control the endpoint of the processes in order to produce layers having known thicknesses. The present invention is directed to addressing these drawbacks.